Looks like 10Wxx is good for 10*F (-12*C) and warmer, and 20Wxx is good for 40*F (-5*C) and warmer. As long is the oil is thin enough to flow when cold it will be thin enough to flow when hot. Maybe too thin when hot. I expect engines that crap out with 20W50 don't flow any better when cold than 20W40, and it's the 20W that is the problem, not the 40 or 50.

I have seen 20wt that would not pour out of bottle. In winter cold. It was like cold honey. Slow sticky goo. The motor will still start but you get plenty wear until it warms up.

Single grade oil is a single viscosity oil. It is gummy when too cold and thin when too hot. Its the right thickness at one temp. That temp can be low or high or middle.

The lower number of a multiweight oil is the singleweight equivalent cold, and the higher number is singleweight equivalent hot.

0-40 is very thin cold, so it pumps easy on startup, but resists excessive thinning at high temps like a straight 40 wt.

Synthetics are superior from this angle, to start with. They are naturally multiweight, i.e. if the world had started off with syn oil, the temperature range indicators would not have been based on single weight numbers.

They don't boil off and turn to ash and sludge like dino oil., a the upper end of dino oil range.

WarrenI thought some more about your thesis that 20W50 might be too thick when hot, and I don't think that's possible. If you look at the viscosity numbers in the articles that i referenced, all the oils we discussed, including 20W40 and 20W50 (synthetic and conventional) are extremely thin at and above 100C, compared to their low temperature numbers. If the additional viscosity of 20W50 in any way limited it's flow to the cam area, then the oil flow to this area would be very seriously compromised during warmup.

The problem with 20W50 oil, is when the engine is cold, and is the same problem with using 20W40 when cold. The dealer was probably either:

1) refering to the use of 20WXX oil, at lower ambient temperatures, when 10WXX is specified, or2) He was simply using your use of 20W50 as an excuse for the cam failure.TW-871987 TW200, 15/44 sprockets, DID O-ring chain, Award winning, "Quick Release" Action Packer :-))